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Technology : Can’t see the tanks for the trees

By Chris Watkins

SHOOTING your allies isn’t the best tactic in peace or war, but “friendly
fire” accidents are all too common and in fact accounted for nearly 20 per cent
of the 615 allied casualties sustained in the Gulf War.

Now researchers at the University of St Andrews in Scotland are working to
make such accidents a thing of the past. Their combat identification system
(CID) is somewhat bizarre: it lets friendly forces recognise each other without
revealing themselves to the enemy by using electromagnetic radiation that makes
tanks “look” like hot rocks and trees.

For years, NATO has been looking for a cheap and reliable way to let its
troops recognise each other. Existing systems often rely on vehicles constantly
emitting radio signals or interrogating each other before an attack to find out
if a potential target is friend or foe. But the brief radio transmissions used
to exchange messages also advertise their presence to the enemy. This means that
tanks and personnel carriers often have to switch off their identifiers before
entering hostile territory.

St Andrews has been working with Britain’s Defence Research Agency to develop
the system, which the electronics giant GEC built in time for NATO to include
last month in tests of competing CIDs now being developed in Britain, France,
Germany and the US.

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The competition rules were strict: the identifying signal should only be seen
with an appropriate receiver, captured receivers should become useless, and it
must be cheap enough to fit on all gun-bearing vehicles. The best performing
system is likely to be adopted by the armed forces of the competing nations.

The French, German and American systems rely on radio to identify vehicles.
The British system was believed to be the favourite because of its unique
approach. Although the trials were held in May, NATO has not yet revealed the
winning system.

The St Andrews system works by mimicking the electromagnetic radiation
naturally given out and reflected by warm objects like trees and rocks. It
broadcasts similar random emissions that contain hidden information.

The system generates a random sequence of numbers with an algorithm and uses
these numbers to adjust the frequency or amplitude of a broadcast. Tanks or
other army vehicles would be fitted with a millimetre-wave emitter coupled to
the random number generator to produce the signal. Millimetre waves fall between
infrared and microwaves in the electromagnetic spectrum, and are emitted by
anything warm that contains water.

The random number generator used is similar to those found in
computers. But instead of producing a statistically random string of numbers, an
identically “random” sequence is generated each time the program is run. If you
know the original algorithm, it is easy to recognise the pattern of numbers. But
without this “key”, says physicist Jim Lesurf of St Andrews, the broadcast
appears to have no pattern at all. An enemy will see only random noise which
looks just like that emitted by warm rocks or trees.

Each friendly vehicle will possess a receiver that knows the key. If a
receiver is captured, friendly forces can easily change the pattern. “You can
change the key as often as you like,” says Lesurf. “The minute you change the
key, having a receiver becomes useless.”

As the identifying signal is effectively camouflaged, it can be
left switched on all the time without drawing unwanted attention. “It doesn’t
attempt to look like one particular thing,” says Lesurf. “It attempts to look
similar to the kind of variations we see in the natural world.”